Generally speaking, a lighting device is often used outdoors, where it tends to become dirty, or indoors with cigarette smoke and odor floating in the air.
Lighting devices used outdoors are especially easy to be contaminated with dirt and debris because of the presence of various atmospheric pollutants, for example those contained in automobile exhaust gas such as CO.sub.2 (carbon dioxide), NO.sub.x (nitrogen oxide) and carbon particles, and oil mist exhausted from a diesel engine vehicle due to incomplete combustion.
As such a lighting device is often disposed high above a road surface or at a dark place in a tunnel, costs for cleaning when contaminated with dirt or debris and other maintenance operations are high.
Lighting devices used indoors tend to be contaminated with nicotine and other impurities.
In that case, too, their maintenance is not always easy, and there is a demand for lighting devices which are easier to maintain.
An example of a means for solving this problem by decomposing adhering substances through oxidation is offered in Japanese Patent Laid-open No. 1989-169866, which discloses a fluorescent lamp hermetically containing mercury to be caused to radiate ultraviolet light by means of negative glow discharge in a translucent envelope, wherein a photocatalytic film made of titania (TiO.sub.2), which is a substance having the photocatalytic function, is formed on the surface of the envelope.
The lighting device described above is adapted to ionize and excite the mercury by means of negative discharge, thereby causing the mercury to radiate ultraviolet with wavelengths of 185 nm and 245 nm so that the radiated ultraviolet would remove the odor and decompose organic substances in the ambient atmosphere or perform other similar functions.
To explain it in more detail, when light having a wavelength range whose energy exceeds that of the band gap, i.e. the width of the forbidden band, of a semiconductor is radiated, electrons and electron holes are generated in the semiconductor, causing a electron-transfer reaction. For example, titania (TiO.sub.2) is a semiconductor having a band gap of approximately 3.0 eV. When so-called ultraviolet light having a wavelength of less than 410 nm contained in the light radiated from a man-made light source such as an incandescent lamp or a high intensity discharge lamp is radiated to titania (TiO.sub.2), electrons and electron holes, i. e. escape holes, are formed in the titania (TiO.sub.2), and movement of these holes causes an electron transfer reaction on the surface of the semiconductor. As these holes have sufficient oxidation capability, in other words a force sufficient to remove electrons equivalent to the energy of the band gap, substances adhering to or otherwise in contact with the surface of the titania (TiO.sub.2) are oxidized and decomposed during the electron-transfer reaction by the oxidation capability of the holes.
As titania (TiO.sub.2) has the characteristic of generating a strong oxidation capability when exposed to ultraviolet light, thereby enhancing oxidation and decomposition of substances adhering to the surface of the titania (TiO.sub.2), such as acetaldehyde, methyl mercaptan, hydrogen sulfide or ammonia, it facilitates cleaning of dirt or debris adhering to the lamp due to atmospheric pollution or other causes. It should be noted that titania (TiO.sub.2) may in some occasions perform the photocatalytic function under visible light having wavelengths of more than 410 nm, because the band gap of the titania (TiO.sub.2) is variable to some extent depending on the density of an impurity.
An example of a lighting device which uses a lamp for general illumination and has a photocatalytic function is offered in Japanese Patent Laid-open No. 1995-111104, which discloses a structure that calls for forming a photocatalytic film on the inner surface of a translucent cover facing the lamp, and producing a photocatalytic action by use of ultraviolet radiated from the lamp, thereby deodorizing and disinfecting the air introduced into the translucent cover.
Although a photocatalytic film is formed on the inner surface of the translucent cover, its purpose lies in deodorization. Therefore, the structure described in Japanese Patent Laid-open No. 1995-111104 remains unproved as to whether it has sufficient cleaning capability.
It is known to those skilled in the art that the activity of a photocatalytic film increases in virtually direct proportion to the thickness of the film if the light radiated to the photocatalytic film remains constant.
Therefore, in cases where a photocatalytic film is formed on the translucent cover of a lighting device in order to prevent it from becoming dirty in an ambience, such as outdoors, which contains a great quantity of dirt and pollutants, the photocatalytic film has to be considerably thick.
It has been found, however, that merely increasing the thickness of the photocatalytic film does not always enhance the decomposition of grime. It has also been confirmed through experiments that the transmittance of visible light in a certain wavelength range decreases due to the interference and action of the photocatalytic film. Therefore, if a photocatalytic film is used together with a light source of which the maximum peak wavelength of the optical output corresponds to a wavelength where the transmittance of the photocatalytic film is low, it may result in a decrease in the illuminating efficiency caused by the reduced illumination intensity or inadequate light distribution.
In cases where a titanium oxide is used as a photocatalytic film, it is a common practice to form the photocatalytic film by using titanium alkoxide. Although a photocatalytic film thus formed has a high transmittance, it sometimes presents an insufficiently low photocatalytic capability. The decrease in the photocatalytic capability is caused by a decrease in the quantity of the anatase-type titanium oxide, which has a highly effective photocatalytic capability, contained in the photocatalytic film. For this reason, it is necessary to make the photocatalytic film thicker even at the cost of reducing the transmittance to some extent.
However, as the refractive index of titanium oxide is relatively high, a thick photocatalytic film often causes interference between visible light which will produce a rainbow-color interference pattern.
A measurement of the photo-absorption spectrum indicates that a lighting device offered in the aforementioned Japanese Patent Laid-open No. 1995-111104, too, has such drawbacks as a low transmittance of visible light, which results in a reduced efficiency, as well as appearance of photo-interference peak waveforms.
Such interference colors as mentioned above are undesirable, because they have an unfavorable influence on objects targeted for radiation and also impair the appearance of the lighting device itself.
In order to solve the above problems, an object of the present invention is to provide a lighting device of which decrease in the illuminating efficiency is limited, maintenance is easy, and generation of interference colors is prevented without the danger of decrease in its photocatalytic effect or transmittance of visible rays.